1. Field of the Invention
The present invention relates to a distributor type fuel injection pump. More particularly, the present invention pertains to a distributor type fuel injection pump which is designed so that the injection timing is controlled electronically.
2. Description of the Prior Art
Japanese Patent Application Laid-Open Publication (KOKAI) No. 57-97024 (1983) discloses a distributor type fuel injection pump for use in a diesel engine wherein a timer piston is moved in response to the operation of a timing control valve which is subjected to duty control, thereby controlling the injection timing.
In this type of timer, as shown in FIG. 10, a timer piston chamber 2a is formed in a housing and a timer piston 2b is slidably inserted into the timer piston chamber 2a so as to partition it into a high-pressure chamber 20 and a low-pressure chamber 21 which are defined at both sides thereof. In addition, a solenoid-operated timing control valve 3 is attached to an end portion of the housing. The high-pressure chamber 20 is communicated with a pump chamber 9 through an orifice 200 and a port 201 which are bored in the timer piston 2b, and a side portion of the high-pressure chamber 20 is communicated with a high-pressure groove 30 in the timing control valve 3 through a port 300. A spring 2d is disposed in the low-pressure chamber 21 to constantly bias the timer piston 2b toward the high-pressure chamber 20. The low-pressure chamber 21 is communicated with a low-pressure hole 31 in the timing control valve 3 through a passage 32.
In this prior art, a timer piston control oil-hydraulic path is formed with respect to the pump chamber pressure Pt with the orifice 200 bored in the timer piston 2b defined as a boundary, as shown in FIG. 11, so that the fuel oil in the pump chamber 9 flows into the high-pressure groove 30 in the timing control valve 2 through the orifice 200 in the timer piston 2b, the port 201, the high-pressure chamber 20 and the port 300.
In the above-described oil-hydraulic path, it may be considered that the orifice 200 cuts off a portion of the pump chamber 9. It is clear that the smaller the dead volume of the cut-off portion, the better the timing control performance. As the conventional oil-hydraulic path is viewed dynamically, the pressure that actually activates the timer piston 2b is that in the high-pressure chamber 20 and therefore the dead volume V.sub.2 increases by an amount corresponding to the ports 201 and 300, resulting in an increase in the work done by the timing control valve 3 to change the pressure Pt.sub.2 in the high-pressure chamber 20 through leak control. In other words, the amount of work required to change the level of working pressure acting on the timer piston 2b increases and the response of the timer pistion 2b to the injection timing control lowers.